Implement copy_file_range as alternative to ioctl(FICLONE)

[EirikAskheim]

reflink functionality got added to zfs in version 2.2.0. ioctl(FICLONE) is supported when reflinking inside a dataset, but it is possible to reflink between datasets using copy_file_range. copy_file_range will fall back to normal copy if reflink is not possible.

Is it possible to add an option to use copy_file_range when reflinking?

openzfs/zfs#405 (comment)

[NobodyXu]

A PR is welcomed, but with a cavert:

reflink should not fallback to copy, only reflink_or_copy is allowed to do so

[NobodyXu]

There's copy_file_range in rustix, however it seems that we cannot force it to fail on reflink failure.

Maybe we can start by using it in reflink_or_copy first, then figure out how to use it in reflink later?

[EirikAskheim]

Looks like std::fs::copy is already trying copy_file_range, so with reflink_or_copy I guess reflink will fail and the fallback is std::fs:copy which already uses copy_file_range?

I agree that having reflink potentially making a copy on failure is unwanted behavior and using reflink_or_copy is a better option.

[NobodyXu]

Looks like std::fs::copy is already trying copy_file_range, so with reflink_or_copy I guess reflink will fail and the fallback is std::fs:copy which already uses copy_file_range?

If that's the case then it's indeed already implemented.

There's nothing more we can do now, until cooy_file_range adds more flags to disable falling back to copy.